1. Field of the Invention
This invention relates generally to a method and apparatus for automatically maintaining a desirable value of flux and, more particularly, to a method and apparatus for automatically maintaining flux within a flux reservoir associated with a solder machine at a predetermined pH level and/or acid number by automatically titrating samples of the flux, and adding flux or thinner material to the reservoir based on the results of the titration.
2. Discussion of the Related Art
During a typical process of assembling electrical components on a printed circuit board (PCB), the electrical components are first positioned within suitably configured openings through the printed circuit board, and are then later soldered to conductive traces on the printed circuit board to provide the desirable electrical connections between the components. High capacity manufacturing processes that rapidly produce a great quantity of PCBs of this type generally include machines that insert the electrical components onto the PCB, and machines that solder the components to the board. After the electrical components have been positioned on the PCB, but prior to the PCB being subjected to the soldering step, a flux is applied to the PCB by a flux application apparatus within a soldering machine, such as a wave fluxing or foaming machine or an open spray fluxing drum system or mist system. The soldering flux cleans the conductive traces on the PCB in order to improve intermetallic bonding, and thus, provide better electrical connections.
Modern fluxes are either alcohol based or water based, where the alcohol or water makes up about 95% or more of the flux solution, to accommodate environmental concerns. The alcohol or water base of the flux solution is referred to as thinner in this disclosure. The remaining 5% or less of the flux solution is suitable acids and other chemicals depending on the particular flux. For a discussion of a particular low residue soldering flux of this type, see for example U.S. Pat. No. 5,004,509 issued Apr. 2, 1991 to Bristol, assigned to the assignee of the instant application, and herein incorporated by reference. The concentration of acid and thinner in an acid flux can be determined by titration. Titration of an acid solution is accomplished via addition of a measured quantity of a reagent to raise the pH of the sample to a measured level. The pH is the logarithm of the expression 1/(hydrogen-ion concentration). The acid number of the sample solution can then be calculated. The acid number of an acid flux is a function of its normality.
Because the flux provides a mechanism that enables the solder to better adhere to the conductive traces on a PCB so as to provide high quality electrical connections by the solder, it is necessary that the flux be of a preferred mixture, and not be contaminated by outside particulates that can adversely effect the ability of the flux to perform its function. One problem that effects the quality of the flux occurs as a result of particulate contamination. As the flux is applied to the PCBs, particulates from the PCBs are washed back into the reservoir storing the flux. The particulates mix with the flux, as does air and humidity, and act to form a "white residue" on the PCBs. This residue contaminates the conductive traces on the PCBs, and adversely effects their conduction. The "white residue" can be controlled by maintaining the acid number of the flux below a certain value dependent on the type of acid flux.
Additionally, since modern fluxes are about 95% alcohol or water, heat and humidity within the manufacturing environment cause the flux to evaporate and/or humidity to dissolve into the flux. The evaporation of the thinner causes the mixture of flux in the flux reservoir to diverge from the desirable pH level and acid number that provides the best flux solution. It is therefore necessary that some process be undertaken to monitor the pH level and/or acid number of the flux, and add the appropriate quantity of flux and/or flux thinner as necessary depending on this pH level to correct for evaporation.
Currently, the flux solution of applying flux is monitored by periodically taking flux samples from the flux reservoir during the manufacturing process, and testing the flux. Presently this is done at production lines, in a laboratory environment, to determine the pH level and/or acid number of the flux. Depending on the outcome of those tests, appropriate thinner or flux solution is added to the reservoir to replenish the supply and provide the necessary pH level to the flux already present in the reservoir.
The above-described process of maintaining the appropriate flux solution in the flux reservoir of a fluxing machine has a number of drawbacks. These drawbacks are related to the requirement of operator intervention and the necessity to take a significant number of samples. Specifically, analysis of the flux samples requires extensive training to accurately perform the tests to achieve desirable results.
What is needed is a flux monitoring system that automatically determines the flux pH level and corresponding acid number, and automatically adds the appropriate flux or thinner material to the flux reservoir to maintain a desirable pH level and acid number. It is therefore an object of the present invention to provide such a system.